feat: Pi-style iterative planning for OpenCode plugin (#318)

* feat: extract checklist utilities to shared package

Move ChecklistItem, parseChecklist, extractDoneSteps, and
markCompletedSteps from Pi extension to @plannotator/shared
for reuse by the OpenCode plugin.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: Pi-style iterative planning for OpenCode plugin

Rewrite the OpenCode plugin to match Pi's planning methodology:

- Inject rich iterative planning prompt when agent is "plan"
  (explore → update plan → ask user loop, structured plan format)
- File-based submit_plan: reads plan from disk instead of requiring
  it as a string arg. Resolves path from OpenCode's system prompt,
  falls back to PLAN.md
- Suppress plan_exit via tool.definition hook (directs to submit_plan)
- Add PLANNOTATOR_ALLOW_SUBAGENTS env var for #289
- Pass planFilePath to denial feedback template
- Keep existing subagent/build/title guards intact

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address review findings in OpenCode plugin

- Cache agents list (static per session, was fetched every LLM call)
- Remove TOCTOU: drop redundant file.exists() before file.text()
- Eliminate duplicate system.join() by reusing joined string
- Resolve getSharingEnabled() once in submit_plan instead of twice
- Use path.join() instead of string concatenation for file paths

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: session-scoped plan files and tool-neutral prompt

Replace plugin-global resolvedPlanFilePath and prompt-path parsing with
per-session plan files at ~/.plannotator/session-plans/opencode/{id}/plan.md.
Remove resolvePlanFilePath() regex (fixes spaces-in-path and cross-session
race). Make planning prompt tool-neutral (no write/edit references) so it
works with apply_patch models. Strip OpenCode's native read-only and
experimental-mode prompt lines before injecting Plannotator's planning prompt.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: directory-based plan storage with path validation

Replace session-scoped plan files with a shared plan directory at
~/.plannotator/session-plans/opencode/. Agent picks the filename,
submit_plan takes a path arg and validates it (absolute, inside plan
dir, exists, readable, non-empty) with canonical path checks to
defeat traversal and symlink escapes. Remove summary and plan string
args. Planning prompt is now tool-neutral and directs the agent to
reuse the same file on feedback, not create new ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: use OpenCode's native plans directory for permission compatibility

OpenCode's plan mode permission ruleset only allows edits to
.opencode/plans/*.md and $XDG_DATA_HOME/opencode/plans/*.md.
Our custom ~/.plannotator/session-plans/opencode/ path was blocked
by PermissionDeniedError at the tool level regardless of prompt
stripping. Switch to the XDG path that OpenCode already allows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: exploration-first planning prompt

Restructure the planning prompt as a phased workflow:
Explore → Ask → Write → Submit. The agent now explores the
codebase before creating a plan file or asking questions,
producing better-researched plans for existing codebases.
Greenfield tasks can skip straight to questions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: fix stale path in JSDoc header comment

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: backnotprop